In general, DLC without incorporated hydrogen has considerable interest, due to its higher sp.sup.3 fraction compared to its hydrogenated counterpart; see, for example, V. S. Veerasamy et al., Solid-State Elec. 37,319 (1994); D. R. Mckenzie et al., Thin Solid Films 206,198 (1991); and K. K. Chan et al, Thin Solid Films 212,232 (1992). DLC film possessing negative electron affinity ("NEA") characteristics, have great potential in their application as electron emitters in vacuum microelectronics and have, as such, attracted extensive studies; see, for example, M. W. Geis et.al, IEEE ED Lett. 12,456 (1991); N. S. Xu et al., Electron. Lett., 29,1596 (1993). The interest for the DLC as an emission material results from its unique emission properties; low-field cold emission and emission stability. In addition, the excellent thermal conductivity of the DLC anticipates high maximum currents to be obtained from the DLC film coated emitters. As could be known from these references, DLC film is typically used as an field emitter material for a Field Emission Display ("FED"). In general, FED comprises emission cathodes and a glass face plate coated with phosphors on transparent conducting oxide. In operation, the face plate is held at a high positive voltage. When a color element is addressed, electrons from a cold cathode array bombard the corresponding phosphor element to produce a bright light in the same manner as in a conventional television picture tube. At the present time, diamond, DLC and, crystalline Si, metals such as Mo are mostly adopted as materials for the FED tip. However, using of metals or Si as tip material has the problems such as low durability, high driving voltage due to high work function compared with DLC or diamond. Further, dispersion of electron and particles causes difficulty in maintenance of high vacuum below 10.sup.-7 Torr, as well as oxidization of the tip. The chemical inertness, the hardness, and especially the low work function of the DLC make it an excellent electron emitting material for the FED.
Cold cathode electron emitters obtained by depositing diamond films on Si tips, Mo tips, or W tips are also widely discussed. The field strength needed for electron emission has been reduced to less than 3.times.10.sup.4 V/cm, which is substantially lower than the field strength required in the conventional metal tips Field Emission Array ("FEA"), as &gt;1.times.10.sup.6 V/cm.
Heretofore, the DLC film deposited by PECVD method has higher hydrogen content, typically higher than 20 at. %; see, A. Dekempeneer et al., Thin Solid Films 217,56 (1992). The incorporated hydrogen reduces sp.sup.3 fraction in the film, resulting in reduction of film hardness. Hydrogen free DLC can be obtained by filtered vacuum arc deposition or by ion beam deposition; see for example, S. Aisenberg et al., Appl. Phys. 42,2953 (1971). Mckenzie et al., deposited hydrogen free DLC with more than 85% sp.sup.3 fraction by a filtered vacuum arc deposition; see, Mckenzie et al., Phys. Rev. Lett. 67,773 (1991). A typical filtered vacuum arc discharge method is characterized in depositing carbon ions formed by arc discharge by applying magnetic field and electric field to said carbon ions. However, it is not easy to get a uniform deposition in large area by this method.
FIG. 1 shows a prior art process for DLC layer forming by using PECVD method. Deposition plasma comprising methane (other carbon hydrides gases can also be used), hydrogen and helium is used in this method. However, in this method, radicals containing hydrogen are formed in the course of decomposing the methane which is included in the deposition plasma, causing a subject sample to inevitably contain hydrogen.